Band-structure tailoring and surface passivation for highly efficient near-infrared responsive PbS quantum dot photovoltaics

Journal of Power Sources 333 (2016) 107e117

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Band-structure tailoring and surface passivation for highly efficient near-infrared responsive PbS quantum dot photovoltaics Ru Zhou a, ***, Haihong Niu a, Fengwei Ji a, Lei Wan a, Xiaoli Mao a, Huier Guo a, Jinzhang Xu a, **, Guozhong Cao b, * a b

School of Electrical Engineering and Automation, Hefei University of Technology, Hefei, 230009, Anhui Province, PR China Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Highly efficient NIR responsive PbS QD photovoltaics have been build.
A superior PCE of 4.08% is reached for hybrid (Pb,Cd)S/CdS configuration.
The work shows positive roles of band-structure tailoring and surface passivation.
Band-structure tailoring is performed for desired charge injection.
Surface passivation is conducted for suppressed charge recombination.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 July 2016 Received in revised form 2 September 2016 Accepted 28 September 2016

PbS is a promising light harvester for near-infrared (NIR) responsive quantum dot (QD) photovoltaics due to its narrow bulk band gap (0.41 eV) and large exciton Bohr radius (18 nm). However, the relatively low conduction band (CB) and high-density surface defects of PbS as two major drawbacks for its use in solar cells severely hamper the photovoltaic performance enhancement. In this work, a modified solutionbased successive ionic layer adsorption and reaction (SILAR) utilizing mixed cationic precursors of Pb2þ and Cd2þ is explored, and such a scheme offers two benefits, band-structure tailoring and surface passivation. In-situ deposited CdS suppresses the excessive growth of PbS in the mesopores, thereby facilitating the favorable electron injection from PbS to TiO2 in view of the up-shifted CB level of QDs; the intimate interpenetration of two sulfides with each other leads to superior passivation of trap state defects on PbS, which suppresses the interfacial charge recombination. With the construction of photovoltaics based on such a hybrid (Pb,Cd)S/CdS configuration, impressive power conversion efficiency up to 4.08% has been reached, outperforming that of the conventional PbS/CdS pattern (2.95%). This work highlights the great importance of band-structure tailoring and surface passivation for constructing highly efficient PbS QD photovoltaics. © 2016 Elsevier B.V. All rights reserved.

Keywords: Solar cell Quantum dot PbS Electron injection Charge recombination

1. Introduction * Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (R. Zhou), [email protected] (J. Xu), [email protected] (G. Cao). http://dx.doi.org/10.1016/j.jpowsour.2016.09.160 0378-7753/© 2016 Elsevier B.V. All rights reserved.

The pressing demand of high efficiency and cost-effective photovoltaics for clean and sustainable energy becomes an important concern nowadays [1e3]. Quantum dot solar cells (QDSCs) utilizing inorganic quantum dots (QDs) as the photon absorbers have

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received tremendous attention in view of multiple advantages of QDs, such as large extinction coefficient, tunable band gap, high stability and fascinating multiple exciton generation (MEG) effect [4,5]. The MEG fingerprint permitting a quantum yield greater than 100% enables QDSCs to deliver power conversion efficiency (PCE) up to 44% in theory, far exceeding the Shockley-Queisser limit [6]. A range of QDs have been explored as sensitizers for light harvesting, including CdS [6], CdSe [7], PbS [8], CuInS2 [9], alloyed Zn-Cu-In-Se [3], etc. In comparison with the recent research hotspot of metal halide perovskite materials in the photovoltaic community, which has offered PCEs exceeding 20% but suffered from moisturesensitive nature, inorganic QDs exhibit excellent stability under ambient environment [10e12]. One of the most salient challenges of QDSCs is to expand the light absorption range from the visible to the near-infrared (NIR) region of the solar spectrum, thereby increasing the photocurrent density [4,13]. The semiconductors of lead compounds with narrow band gaps in the infrared region (e.g., PbS: Eg ¼ 0.41 eV; PbSe: Eg ¼ 0.28 eV) and large excitation radii (e.g., PbS: r ¼ 18 nm; PbSe: r ¼ 46 nm), have attracted enormous interest as sensitizers as they allow the extension of absorption band toward the NIR part and the great potential to realize the MEG effect [4,13]. Benefiting from these merits, the use of lead-series QDs in solar cells has demonstrated the great potential for solar energy conversion [14e16]. Therefore, PbS has been shown to be a fascinating absorbing material, delivering impressive photocurrents and PCEs [17e19]. In particular, two architectures for PbS QD photovoltaics prevail recently: one includes a photoelectrode employing QDs to form a pn heterojunction (two dimensional); the other possesses a QDsensitized nanocrystalline TiO2 photoelectrode (three dimensional) [20]. The former configuration is generally fabricated using an ex-situ method, in which the pre-synthesized monodispersed QDs are anchored onto semiconductor oxides to produce a planar junction. In such a scenario, the photo-generated carriers deep inside the QD active layers have to undergo a long-distance transport via numerous hopping processes before being separated at the junction, which accordingly limits the thickness of QD layer for sufficient light harvesting [4]. The ex-situ method has also been used to assemble the QDs for the latter configuration; however it is difficult to achieve uniform and sufficient coverage of QDs on TiO2 in view of the bulky nature of linker-capped colloidal QDs. On the contrary, another widely used strategy named in-situ growth is conducive to high loading of QDs and intimate contact of TiO2 and QDs with each other [7]. Therefore, it is anticipated that such a three dimensional sensitization configuration with in-situ growth of QDs benefits highly efficient PbS QD photovoltaics by virtue of sufficient light harvesting, quick charge transport and efficient charge collection. Unprecedentedly high photocurrents exceeding 30 mA cm2 have been achieved for PbS QDSCs, reflecting the great contribution of NIR responsive PbS active layer [18,21]. However, PbS as a sensitizing material has been reported to show two major drawbacks for its use in solar cells: 1) the relatively low conduction band (CB) of QDs prohibits efficient electron injection from PbS to electron-transporting materials (e.g., TiO2) [22], and 2) the high density of trap state defects (~1017 cm3) on PbS QDs gives rise to severe interfacial charge recombination [23]. Both disadvantages severely hinder the enhancement of photovoltaic performance. The first problem could be circumvented by reducing the particle size of PbS to up-shift the CB energy level, and thus a low concentration precursor solution is usually preferred for PbS deposition because of the need for better control over successive ionic layer adsorption and reaction (SILAR) to avoid the excessive growth of large QDs with lowered CB edge [18e21]. Ligand-assisted surface passivation is an effective strategy to combat the second problem, i.e., the charge recombination losses, and a batch of short-

chain organic linkers (e.g., mercaptopropionic acid, ethanedithiol) and inorganic halide anions (e.g., Cl, Br, I) have been developed for this purpose [24,25]. Therefore, the exploitation of more effective strategies contributing the facilitated electron injection and suppressed charge recombination is of great importance [26,27]. In this work, NIR responsive PbS QD photovoltaic devices have been fabricated through a facile solution-based in situ strategy at ambient conditions, i.e., a modified SILAR method. A new hybrid configuration of (Pb,Cd)S/CdS, different from the conventional PbS/ CdS structure [22,28], is constructed to perform band-structure tailoring and surface passivation for improving the photovoltaic performance. In our scheme, the simultaneous deposition of CdS prevents the excessive growth of PbS QDs in the mesopores, upshifting the CB level of PbS, and the intimate interpenetration of two sulfides leads to an effective passivation of trap state defects on PbS QDs. Highly efficient QDSC based on (Pb,Cd)S/CdS configuration exhibiting a PCE up to 4.08% has been achieved, thanks to favorable injection of photogenerated electrons and superior suppression of charge recombination. 2. Experimental section 2.1. Chemicals and materials Titanium oxide (TiO2, Degussa, P25), terpineol (C10H18O, Sinopharm), ethyl cellulose ([C6H7O2(OC2H5)3]n, Sinopharm), cadmium acetate dihydrate (Cd(CH3COO)2$2H2O, 98%, Sinopharm), titanium tetrachloride (TiCl4, 98.0%, Sinopharm), lead acetate trihydrate (Pb(CH3COO)2$3H2O), 99.5%, Sinopharm), sodium sulfide nonahydrate (Na2S$9H2O, 98.0%, Sinopharm), sulfur (S, purified by sublimation, 99.5%, Sinopharm), zinc acetate dihydrate (Zn(CH3COO)2$2H2O, 99.0%, Sinopharm), brass foil (alloy 260, 0.3 mm thick, Alfa Aesar), hydrochloric acid (HCl, mass fraction ¼ 36.5e38.0%, Sinopharm) and methanol (CH3OH, 99.5%, Sinopharm) were all used as received. 2.2. Preparation of mesoporous TiO2 films Commercially available F-doped tin oxide glass (FTO, sheet resistance ~ 15 U sq_1) was used as transparent conducting substrates to prepare TiO2 photoelectrodes. For the preparation of TiO2 paste, 0.5 g Degussa P25 mixed with 0.25 g ethyl cellulose and 1.75 g a-terpineol were first dispersed into 5.0 mL ethanol, and then sonicated for 30 min to form a slurry after removing the ethanol under stirring. The photoelectrodes were prepared by doctorblading of the as-prepared TiO2 paste on the TiCl4 aqueous (50 mM) pretreated cleaned FTO substrate, followed by sintered at 500  C for 30 min in a Muffle furnace (in air) with a heating rate of 5  C min1. The post-treatment with the aqueous solution of TiCl4 and another calcination of 500  C were carried out before the sensitization of QDs. The thickness of the TiO2 film, measured from the cross sectional image of SEM, was ca. 12 mm, as shown in Fig. 2c. 2.3. In situ assembling of QDs by SILAR The SILAR processes were conducted for in situ assembling of PbS, CdS and hybrid systems of (Pb,Cd)S, (Pb,Cd)S/CdS, and PbS/CdS in this work. For a typical SILAR process, the films were sequentially immersed into the as-prepared solutions of cationic and anionic precursors to allow the ions of the reactants to penetrate into the mesoporous film and incorporate into the interior of mesopores, leading to the formation of in situ-grown QDs. Specifically, the deposition of PbS, CdS and (Pb,Cd)S QDs includes the use of three kinds of cationic methanol precursors, i.e., 0.02 M Pb(CH3COO)2, 0.1 M Cd(CH3COO)2 and mixed solutions of concentration-fixed

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0.02 M Pb(CH3COO)2 and 0.02x M (x ¼ 0.1, 0.5, 1, 5 and 10) Cd(CH3COO)2, respectively. A single SILAR cycle herein consisted of 1 min dip-coating of the as-prepared TiO2 photoelectrode into each cationic solution mentioned above and subsequently into anion solution (0.1 M Na2S dissolved in the solvent of water and methanol with the volume ratio of 1/1, v/v). Following each immersion, rinsing was undertaken using methanol to remove the excess of each precursor solution and dried in air for several minutes. A certain number of SILAR cycles were employed to obtain a desired amount of the corresponding QDs loaded on TiO2 films. All the photoelectrodes were ultimately coated with two SILAR cycles of ZnS passivation layer, by dipping alternatively into 0.1 M Zn(CH3COO)2 and 0.1 M Na2S solutions for 1 min/dip. 2.4. Solar cell fabrication Solar device was assembled by sandwiching the as-prepared photoelectrode and Cu2S counter electrode using a scotch tape spacer (ca. 50 mm thick) and permeating the assembly with the polysulfide electrolyte [29,30]. The polysulfide electrolyte employed in this study was composed of 1 M S and 1 M Na2S in deionized water. The counter electrode was a Cu2S film fabricated on a brass foil, and the preparation procedure can be described briefly as follows: a brass foil was immersed into HCl solution (mass fraction ¼ 36.5e38.0%) at ca. 70  C for about 0.5 h, then rinsed with water and dried in air; the etched brass foil was then dipped into the as-prepared polysulfide electrolyte for about 5 min, resulting in a black Cu2S layer forming on the foil. 2.5. Characterization of materials and solar devices Morphologies of the film samples were directly characterized by a scanning electron microscope (SEM, SU8020). Transmission electron microscope (TEM) and high-resolution TEM observations were performed on a JEM-2100F microscope equipped with an EDAX system (Oxford 7788) to analyze the element content and distribution. Raman spectroscopy was obtained on a confocal Raman spectrometer (HR Evolution) under the laser excitations of 532 and 633 nm. XRD measurements were performed on an X'Pert PRO MPD diffractometer (Panalytical B. V.) using the Cu Ka as the irradiation. XPS spectra were collected on an ESCALAB220Xi electron spectrometer from Thermo Scientific. Optical absorption spectra were measured on an Agilent UVeviseNIR spectrophotometer (CARY 5000) fitted with an integrating sphere accessory. Current density-voltage (J-V) characteristics of solar device with an active area of ~0.196 cm2 were carried out under AM 1.5 (100 mW cm2) illumination, which was generated by a solar simulator (Oriel Sol 3A Solar Simulator, USA) liked with a digital source meter (Keithley, 2400). Electrochemical impedance spectroscopy was recorded on an Autolab320N electrochemical workstation. The frequency range explored was from 100 kHz to 1 Hz, and the parameters of the equivalent circuit were fitted by using Zview software. 3. Results and discussion Mesoporous TiO2 films were filled up with in situ SILAR-grown QDs to construct photoelectrodes. Several sensitizing configurations have been analyzed, including single systems of PbS and CdS, and hybrid systems of (Pb,Cd)S, PbS/CdS and (Pb,Cd)S/CdS. Herein, the samples are labeled as PbS(m) and PbS/CdS for example, where m refers to the numbers of SILAR cycles and the latter pattern represents CdS being further coated on the pre-deposited PbS. A series of (Pb,Cd)S QDs sensitized TiO2 films were prepared by four SILAR cycles under a concentration-fixed 0.02 M Pb2þ solution and

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an increasing feed Cd2þ molar concentration of 0, 0.002, 0.01, 0.02, 0.1 and 0.2 M, and the resulted samples are hereafter denoted as Pb2þ/Cd2þ ¼ 1/0, 1/0.1, 1/0.5, 1/1, 1/5 and 1/10 samples, respectively, according to the corresponding molar ratios of precursors. First of all, it is of great importance to clarify whether the socalled (Pb,Cd)S QDs obtained belong to a ternary alloyed compound (PbxCd1xS solid solution) or a mixture of phases [(CdS)x(PbS)1x] [31,32]. Herein PbS, CdS and PbS/CdS sensitized TiO2 films were taken as the control samples, and a large number of SILAR cycles were operated to enhance the detecting signals of QDs relative to TiO2. Raman spectroscopy is a powerful technique for the identification of intrinsic vibronic structures of materials. Fig. 1a and b shows the room-temperature Raman spectra of QDsensitized TiO2 samples under the laser excitations of 532 nm and 633 nm lines, respectively. Several bands resolved as 143, 196, 396, 515 and 638 cm1 are attributed to the Raman spectrogram characteristic peaks of anatase TiO2; in addition, the spectra also display prominent bands with the peaks at 299 and 598 cm1, which can be assigned to the first and second-order longitudinal optical (LO) Raman peaks of CdS nanoparticles. Apparently, the intensities of Raman peaks for CdS are much weaker under the excitation of 633 nm with respect to the 532 nm excitation line [33]. In our experiments, no obvious Raman peaks were detected for PbS, the same as some cases reported in the literatures [34,35]; the reason might be that PbS is a relatively weak Raman scatter at room temperature in view of its high absorptivity, so as to be susceptible to laser-induced degradation when intensely irradiated. Moreover, no evident differences were detected for the Raman bands between the samples of (Pb,Cd)S(16) and PbS(8)/CdS(8); both spectra exhibit the characteristic peaks of CdS without any obvious shift. In consideration of this fact, it is speculated that the as-prepared (Pb,Cd)S deposited on the TiO2 films should be a mixture of PbS and CdS. Fig. 1c presents the XRD patterns for the as-fabricated QDsensitized TiO2 samples. As shown, different from the pure TiO2 pattern, several additional broad diffraction peaks of (111), (200), (220) and (311) as marked in the figure can be resolved after the sensitization of PbS, CdS or (Pb,Cd)S QDs, indicating the successful deposition of the cubic phase of rock salt structure PbS (JCPDS no. 650132) and zinc blende structure CdS (JCPDS no. 800019) onto TiO2 films [31,32,36]. Although the peak intensities for QDs are much weaker in view of their relatively small amount in comparison with the rest of diffraction peaks corresponding to those of TiO2 and FTO substrate, it can be observed that (Pb,Cd)S (the 1/5 sample) exhibits no new diffraction peaks or obvious peak shifts in contrast with pure CdS or PbS, further implying a mixture of phases for (Pb,Cd)S nanocrystals. XPS spectra of (Pb,Cd)S sensitized TiO2 sample as shown in Fig. 1d confirm the co-existence of PbS and CdS. Since the binding energies of S2p for PbS and CdS are quite close to one another, it is difficult to differentiate these two sets of S2p spectra for both metal sulfide separately. Based on these results, it is concluded that the as-prepared (Pb,Cd)S QDs deposited on TiO2 films by SILAR are a mixture of phases [(CdS)x(PbS)1x], rather than a ternary alloyed compound (PbxCd1xS solid solution). This finding is in a good agreement with the binary phase diagram of PbSCdS [32], which describes a negligible solubility between the two phases since the binary system belong to the phase separating case at room temperature (i.e., a near impossibility to form the PbSCdS solid solution phase). The inference will be further supported by TEM and UVeviseNIR absorption spectra below. The plane-view and cross-section SEM images of the bare mesoporous TiO2 substrate and (Pb,Cd)S (the 1/5 sample) sensitized film sample are shown in Fig. 2. It is observed that the bare TiO2 film as presented in Fig. 2aec exhibited a highly porous nanostructure; such mesoporous structure allows the penetration of cationic and anionic precursor solutions, and would prohibit the

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Fig. 1. (a,b) Raman spectra of as-achieved QD-sensitized TiO2 samples under the laser excitations of 532 nm and 633 nm lines, respectively, (c) XRD patterns of QD-sensitized TiO2 samples, and (d) XPS spectra of (Pb,Cd)S sensitized TiO2 sample.

Fig. 2. (aec) Plane-view and cross-section SEM images of the bare mesoporous TiO2 substrate, and (d,e) Plane-view SEM images and (f) EDX spectrum based on the surface of (Pb,Cd)S (the 1/5 sample) sensitized film sample.

formation of very large particles of PbS or CdS precipitations. After the sensitization of (Pb,Cd)S QDs, the mesoporous structure of TiO2 film was largely retained as shown in Fig. 2d and e; however, some small pores were filled with QDs, and the nanoparticles on the sensitized films seem to become larger and coarser compared to the bare TiO2. These observations indicate a successful deposition of QDs on the photoelectrodes. The actual molar ratio of Pb2þ/Cd2þ for the 1/5 sample was examined by EDX analysis. The corresponding EDX spectrum is presented in Fig. 2f. As shown, besides two energydispersion peaks belonging to Ti and O, there also exist typical

peaks of Pb, Cd and S for QD-sensitized films. The atomic ratios of elements are as follows: 1.8% Pb, 2.5% Cd, 4.2% S, with Ti and O being the balance. Moreover, it is noteworthy that the feed molar ratio of Pb2þ/Cd2þ in precursor solutions for the 1/5 sample is ~0.20, while the as-achieved QDs deliver a much higher actual ratio, estimated to be ~0.72. Such a deviation between these two ratios reveals a different deposition rate for Pb2þ and Cd2þ to construct the corresponding QDs with S2. A much higher deposition rate (i.e., more rapid nucleation) for Pb2þ might be explained by the relatively lower solubility product constant Ksp for PbS (8.0  1028)

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compared to that of CdS (7.0  1027) [37]. Considering the fairly low solubility product of these two sulfides, the reaction between Pb2þ (or Cd2þ) and S2 should be very fast, agreeing with what has been observed during our experiments. With the aim of imaging the as-prepared (Pb,Cd)S QDs directly on the TiO2 surface and characterizing the heterostructure interface or surface at the atomic level, we further conducted TEM images for (Pb,Cd)S QDs (the 1/5 sample). As shown in Fig. 3ac, the small dots are well separated and distributed in a homogeneous way over the surface of larger TiO2 nanoparticles, similar to some scenarios reported in literatures [19,22,38]. In view of the mesoporous structure of photoelectrode film, a portion of TiO2 surface remains uncovered, and such an incomplete coverage is generally considered to aggravate the charge recombination in the corresponding solar device; however, the low coverage might not be the main factor contributing to the recombination for PbS based photovoltaics, the surface states have also been demonstrated to play an important role in the recombination process considering the high density of defects on the surface of PbS QDs [22]. High-resolution TEM image (Fig. 3b) further revealed that the well-defined PbS QDs are obtainable and the shape was close to hemispherical, facing towards the TiO2 surface. The mean particle size of most SILARprocessed QDs prepared under four cycles was in the range of 3.5e4.0 nm in diameter for PbS, and a slightly smaller value for CdS. The larger size for PbS nanoparticles obtained undergoing the same

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deposition time might be attributed to the stronger cohesion of PbS than that of CdS [39]. Generally, according to measured energy levels, the efficient electron injection from colloidal PbS QDs into TiO2 has been expected only for QD with the diameter below ~4.3 nm [40]. Moreover, the image allowed the identification of lattice fringes with interplanar spacings of ~0.297 nm and ~0.337 nm, correlated with the (200) plane of PbS and (111) plane of CdS, respectively [20,38]. The corresponding lattice fringes for PbS and CdS separately further evidence that the (Pb,Cd)S sample fabricated by SILAR here at room temperature is a mixture of two compounds. However, it is possible that there might exist an amorphous and ultrathin alloying layer at the boundary of PbS and CdS, in view of the structural mismatch between these two sulfides, resulting in poor lattice fringes [32]. The scenario of CdS “coating” on PbS echoes the more rapid nucleation of PbS with respect to CdS during SILAR processes. Fig. 3deh shows the elemental maps displaying the spatial elemental distribution of Pb, Cd, S, Ti and O, indicating that TiO2 is conformally coated by the elements of Pb, Cd and S. The corresponding EDX analysis performed herein reveals an estimated molar ratio of ~0.78 for Pb2þ/Cd2þ, which is approximately in agreement with the value (~0.72) obtained from the EDX spectrum based on the film surface by SEM (Fig. 2f). This result further confirms the homogeneous distribution of QDs onto the TiO2 film in our approach. Fig. 4a shows the UVeviseNIR absorption spectra of a series of

Fig. 3. (aec) TEM images of (Pb,Cd)S QDs (the 1/5 sample) sensitized TiO2 nanoparticles under different magnifications, and (deh) elemental maps displaying the spatial elemental distribution of Pb, Cd, S, Ti and O, respectively.

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Fig. 4. (a) UVeviseNIR absorption spectra, (b) (Ahy)2 vs hy plots, (c) schematic diagram depicting the band-structures (E vs. vacuum) of PbS QDs achieved under different molar ratio of Pb2þ/Cd2þ, and (d) AM1.5G spectrum from ASTM G173-03 reference spectra and concept of tuning the light absorption range by determining the QD size of PbS QDs.

(Pb,Cd)S QDs sensitized TiO2 photoelectrodes prepared under four SILAR cycles. It is evident that the feature of the successively deposited QDs allows the extension of the absorption band toward the NIR part of the solar spectrum. Along with the increase of Cd2þ concentration, the absorption edge continuously shifts to shorter wavelengths; the color of the film samples displays such a trend from dark brown to light yellow. Such a sequence is interpreted as an expended energy band gap of PbS, indicating the reduced QD size. It is noteworthy that, besides the tuning of absorption range, the characteristic absorption shoulders for CdS and PbS appear when Cd2þ concentration reaches a certain value; for instance, an obvious hump at ~480 nm corresponding to the absorption edge of CdS has been observed for the 1/5 sample. This phenomenon further indicates the formation of a mixture of PbS and CdS phases rather than the solid solutions, which agrees with the result concluded in the above section. The effective band gap of PbS QDs can be estimated by extrapolating the linear portion of the (Ahy)2 versus hy plots at A ¼ 0 according to the following equation, which describes the relationship between the optical band gap (Eg) for direct inter-band transition and the absorption coefficient (A) near the absorption edge [29,41]:

ðAhyÞ2 ¼ c hy  Eg




(1)

where y is the frequency, h is Planck constant and c is a constant. The obtained values of effective band gaps and absorption onsets are summarized in Table 1. As shown, the estimated band gaps are all much larger than that of bulk PbS (0.41 eV) in terms of quantum size effect and become increasingly larger from 1.09 to 1.51 eV with the decreasing Pb2þ/Cd2þ molar ratio, correlated with the pronounced blue shift of absorption edge. Schematically depicted in Fig. 4c are the band-structures (Energy vs. vacuum) of PbS QDs obtained under different feed molar ratio of Pb2þ/Cd2þ as revealed by the corresponding absorption spectra. The energy levels and QD

Table 1 Absorption onset, effective band gap and band gap shift of PbS QDs achieved under different feed molar ratio of Pb2þ/Cd2þ. DE is the band gap blue shift with respect to that of bulk. Sample

Absorption onset (nm)

Effective band gap (eV)

DE (eV)

1/0 1/0.1 1/0.5 1/1 1/5 1/10 CdS

1138 1078 1060 961 947 821 508

1.09 1.15 1.17 1.29 1.31 1.51 2.44

a

a b

0.68 0.74 a 0.76 a 0.88 a 0.90 a 1.10 b 0.02 a

PbS (0.41 eV). CdS (2.42 eV).

sizes are deduced and compiled according to our estimated band gaps and relevant references [20,40,42,43]. For example, the QD size of PbS with the band gap of 1.31 eV is estimated to be ~3.7 nm, and the corresponding CB and VB levels are 3.8 and 5.1 eV, respectively. Therefore, the absorption spectrum of solar devices can be readily tuned by determining the QD size of PbS, as illustrated in Fig. 4d; while the QD size is further controlled with the aid of altering the molar ration of Pb2þ/Cd2þ. In our strategy, it is reasonable that such two sulfides experience intimate mixing with each other during the deposition process; the increasing concentration of Cd2þ suppresses the excessive growth of PbS QDs in the mesopores, thereby reducing the QD size of PbS. As expected, PbS exhibits up-shifted CB level with the decreasing QD size, and a higher CB level is expected to facilitate the more efficient electron injection from QDs to TiO2. Therefore, the extension of the light absorption of photovoltaics into NIR region and the flexibility of band-structure tailoring makes our scheme promising for photovoltaic application. The photovoltaic characteristics of a series of PbS QDSCs

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fabricated under different feed molar ratio of Pb2þ/Cd2þ in the cationic precursor are presented in Fig. 5a and b. The short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and PCE (h) are summarized in Table S1. In particular, at least three identical device samples have been fabricated to check the reproducibility of photovoltaic performance for each solar cell studied in this work. As shown, the PbS QDSC employing the 1/0 photoelectrode (without the addition of Cd2þ in the cationic precursor) exhibits a considerable Jsc of 10.15 mA cm2 but quite poor Voc and FF, leading to a low PCE of 1.06%. In comparison, although the pure CdS QDSC shows large Voc of 0.62 V and FF of 0.51, the corresponding PCE is not very high owing to the small Jsc (7.31 mA cm2). It is evident that, with the presence of Cd2þ ions in the precursor, the photovoltaic performances of QDSCs are significantly improved, especially for parameters of Jsc and Voc; the PCE first displays a great increase and then decreases to some degree with the increase of Cd2þ concentration. Particularly, the solar cell made of the 1/5 photoelectrode acquires Jsc of 18.79 mA cm2, Voc of 0.46 V, and FF of 0.36, yielding an optimized PCE of 3.10%. That is, the optimized synergistic combination of PbS and CdS herein has been demonstrated to deliver panchromatic solar cells with enhanced photovoltaic performance beyond the efficiencies of the single constituents. Fig. 5c illustrates the device architectures and charge transfer mechanisms of PbS QD photovoltaics in this study. As demonstrated, under illumination, photons are captured by the PbS harvesters, generating electron-hole pairs that are quickly separated. Those electrons excited to the CB of QDs inject into the CB of TiO2

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(ket expresses the electron transfer rate), and subsequently collected by the FTO conducting substrate; while the holes left in the VB of QDs are reduced by S2 ions in the electrolyte. Thus a closed circuit is formed, thereby generating photocurrent. In particular, besides these charge transfer processes, there also exist some charge recombination pathways (①⑥) in the devices, as illustrated in the schematic diagram (ker presents the electron recombination rate) [7,44]. Among the six pathways, two main ones closely correlated with the trap states lying between the CB and VB of PbS are marked as ① and ④, which describe the trapping of electrons in the CB of QDs and TiO2 by defects. It is well known that the trap state surface defects usually serve as charge recombination centers to promotes nonradiative recombination when carriers transport in the photoelectrodes, and thus deteriorate the photovoltaic performance, especially the photovoltage [4,24,45]. The ratio of the number of charge carriers collected at the conducting substrate to the number of incident photons (IPCE, also known as the external quantum efficiency) of a solar cell is mainly determined by three parameters of LHE(l), Фinj and hc (herein LHE(l) is the light harvesting efficiency at a certain wavelength, Фinj is the electron injection efficiency, and hc is the electron collection efficiency from the electron conductor into the conducting substrate) [46]. Therefore, for our PbS QD photovoltaic possessing broad light harvestings, on one hand, a favorable band alignment is required for efficient electron ejection between PbS and TiO2 considering the band-structures of TiO2 and QDs; on the other hand, an effective passivation of surface defects on PbS is conducive to the reduction of charge recombination in terms of high-density trap states. In our

Fig. 5. (a) Current density-voltage (J-V) characteristics and (b) photovoltaic parameters (PCE, Jsc, Voc and FF) of PbS QDSCs fabricated under different feed molar ratio of Pb2þ/Cd2þ in the cationic precursor, measured under the illumination of one sun (AM 1.5, 100 mW cm2), and (c) Schematic illustration of device architectures and charge transfer mechanisms of PbS QD photovoltaics in this study (solid purple lines refer to electron transfer processes, whereas dotted red lines show electron recombination pathways). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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scheme, the utilization of Cd2þ ions for the resultant CdS would produce two impressive benefits to meet the above-mentioned demands: 1) up-shifting the CB of PbS by controlling the QD size, thereby facilitating the efficient electron injection from QD to TiO2, as illustrated in Fig. 4c; 2) providing effective passivation of surface defects for PbS through the CdS passivation layers, which echoes the increasingly smaller dark current obtained with the increasing Cd2þ concentration as reflected by J-V curves measured under dark for various devices (Fig. S1). Herein the resultant improved electron injection efficiency and recombination suppression effect contribute to the substantial increase of Jsc and Voc. Meanwhile, it has been shown that the increasing concentration of Cd2þ ions in the cationic precursor decreases the size of PbS QDs, thereby narrowing the light absorption range and reducing the light harvest to a certain degree. The successful balance of these positive and negative impacts gives rise to the achievement of an excellent photovoltaic performance. To sum up, the panchromatic light absorption is significant to boost the photon harvesting; the bandstructure tailoring and surface passivation of PbS QDs both play great roles for the performance enhancement. With the construction of solar cells employing the so-called hybrid (Pb,Cd)S QDs, impressive cell performances are obtainable relative to those of the single systems of PbS or CdS. Based on the discussions above, it is suggested that the utilization of an appropriate amount of Cd2þ ions in the cationic precursor is conducive to the performance improvement of PbS QD photovoltaics. To further enhance the photovoltaic performance and improve the device stability, QDSCs utilizing the hybrid system of (Pb,Cd)S/CdS were also prepared. The additional CdS capping layer is expected to impair the surface defects more effectively. Yang et al. ever attributed the performance improvement of PbS/CdS configuration to the formation of cascading band-structure; however, the photocurrent enhancement extended over the entire spectral range of PbS absorption, rather than the wavelength range of CdS, implying the presence of other mechanisms [47,48]. Therefore, a more probable explanation has been proposed to be the effective surface passivation of PbS with respect to severe charge recombination [47,48]. It is noted that the (Pb,Cd)S mentioned below refers to the 1/5 sample, i.e., with the feed molar ratio of Pb2þ/Cd2þ ¼ 1/5. The photovoltaic characteristics of the corresponding hybrid QDSCs are compared in Fig. 6a and b, and the extracted parameters are tabularized in Table 2. Compared to above-mentioned (Pb,Cd)S cells, (Pb,Cd)S/CdS ones indeed lead to improved PCEs further, as a result of the increased values of Voc and FF. It is well known that a reasonable amount of QDs deposited gives rise to an excellent performance. Specifically, a small amount of QDs would result in a low photocurrent density and a poor PCE, while an overload of QDs

Table 2 Photovoltaic parameters of our hybrid (Pb,Cd)S/CdS cells and the conventional PbS/ CdS one. Device

Jsc (mA cm2)

Voc (V)

FF

(Pb,Cd)S(2)/CdS(4) (Pb,Cd)S(4)/CdS(4) (Pb,Cd)S(6)/CdS(4) PbS(4)/CdS(4)

17.56 19.44 17.14 17.34

0.50 0.50 0.48 0.43

0.44 0.42 0.42 0.40

(±0.42) (±0.51) (±0.46) (±0.52)

(±0.02) (±0.03) (±0.02) (±0.03)

h (%) (±0.03) (±0.02) (±0.02) (±0.02)

3.86 4.08 3.42 2.95

(±0.18) (±0.20) (±0.15) (±0.23)

might also lead to a poor cell performance, possibly owing to the excessive blocking of the mesopores in the TiO2 films [6,7]. Therefore, the PCE further increases and then decreases along with the extended SILAR cycles; the cycle number for (Pb,Cd)S was optimized to be around four, along with further growth of another four layers of SILAR-processed CdS, and the corresponding device acquires Jsc of 19.44 mA cm2, Voc of 0.50 V, and FF of 0.42, yielding an impressive PCE of 4.08%. We would like to pay particular attention to the comparison between such (Pb,Cd)S/CdS cells and the control PbS/CdS cell. As shown, the photovoltaic characteristics of (Pb,Cd)S/CdS cells outperforms those of the conventional PbS/CdS one (h ¼ 2.95%), which also has the aid of CdS capping layer. The significant performance enhancement is mainly attributed to the considerable promotion of Voc and Jsc. The electrochemistry impedance measurements have been carried out to evaluate the interfacial charge recombination processes in solar devices. The impedance spectra shown in Fig. 7a and b were recorded under dark condition at an applied forward bias of 0.50 V, and the curves were fitted in terms of the equivalent circuit as depicted in the inset. It is evident that both Nyquist plots exhibit three semicircles. The contact series resistances (Rs) reflected by the starting point of the curve represent the sheet resistance of FTO and contact resistances between FTO and TiO2; the first semicircles (R1) in the high-frequency range account for the resistance of the redox reaction at the counter electrode/electrolyte interface; the second semicircles (R2) in the intermediatefrequency range correspond to the charge recombination resistance at the TiO2/QD/electrolyte interface; the third semicircles (R3) in the low-frequency range present the resistance of the transport of ions in the electrolyte [7,19,49e51]. The fitted parameters are summarized in Table S2. In consideration of the same polysulfide electrolyte and Cu2S counter electrode used in our work, R1 and R3 both exhibit no obvious differences for PbS(4)/CdS(4) and (Pb,Cd) S(4)/CdS(4) devices. Herein we would like to focus on the most appreciable difference between the devices, i.e., the recombination resistance R2, which reflects the charge recombination processes occurred at the TiO2/QD/electrolyte interface as illustrated in

Fig. 6. (a) Current density-voltage (J-V) characteristics and (b) a comparison of photovoltaic parameters (PCE, Jsc, Voc and FF) of our hybrid (Pb,Cd)S/CdS cells and the conventional PbS/CdS one, measured under the illumination of one sun (AM 1.5, 100 mW cm2).

R. Zhou et al. / Journal of Power Sources 333 (2016) 107e117

115

Fig. 7. (a) Nyquist plots and (b) Bode plots of impedance spectra recorded under dark condition at an applied forward bias of 0.50 V (the inset displays the corresponding equivalent circuit), (c) Current density-voltage (J-V) characteristics measured under dark conditions, and (d) Stability test for (Pb,Cd)S/CdS and PbS/CdS cells.

Fig. 5c. As given in Table S2, the R2 of (Pb,Cd)S(4)/CdS(4) device is 34.02 U, which is about twice that of PbS(4)/CdS(4) device (16.26 U). The result revealed that, compared to PbS(4)/CdS(4) photoelectrode, carriers in the (Pb,Cd)S(4)/CdS(4) photoelectrode are much more difficult to recombine with the redox couple (S2/

S2 n ) in the electrolyte regarding the smaller R2, which signifies a lower recombination rate. As the same TiO2 film, electrolyte, and counter electrode were employed in both cells, the value difference of R2 should be closely correlated with the loading feature of QDs on the TiO2 surface, which might give rise to different charge transport

Fig. 8. A comparison of TEM images between PbS QDs (a) and (Pb,Cd)S QDs (b) sensitized TiO2 nanoparticles, and schematic diagrams showing a more favorable band alignment facilitating the electron injection and a more effective defect passivation effect reducing the charge recombination in photovoltaic devices based on the conventional TiO2/PbS/CdS configuration (c, e) in comparison with our TiO2/(Pb,Cd)S/CdS one (d,f), respectively.

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and recombination characteristics. As expected, the effective surface passivation of PbS achieved by the (Pb,Cd)S(4)/CdS(4) scheme leads to the decrease of defect centers, giving rise to the suppressed charge recombination and promoted charge transport in solar devices. Fig. 7b shows the Bode plots of the impedance spectra. The electron lifetime (tn) in the TiO2 can be evaluated by the peak frequency at the minimum phase angle in the Bode plot according to the equation below [29,50,51]:

tn ¼

1

upeak

1 ¼ 2pfpeak

(2)

As listed in Table S2, the estimated electron lifetime of (Pb,Cd) S(4)/CdS(4) cell is about 9.95 ms, much longer than that of 5.66 ms for PbS(4)/CdS(4) cell. Evidently, the long-lived charge carrier indicates the suppressed interfacial charge recombination in the photoelectrode, and ensures the efficient collection of electrons at the FTO substrate. Moreover, the inference based on impedance analysis is further supported by the J-V characteristics measured under dark conditions as given in Fig. 7c, which displays a smaller dark current for the (Pb,Cd)S(4)/CdS(4) device than that of PbS(4)/ CdS(4) one. In addition, the stability test of solar devices measured at interval time (Fig. 7d) indicates our (Pb,Cd)S(4)/CdS(4) cell ensures a better device stability with respect to the conventional PbS(4)/CdS(4) one. A slight increase of PCEs at the initial stage of the photovoltaic measurements might be derived from the complete diffusion of the electrolytes among the mesoporous photoelectrodes. Therefore, it is concluded that (Pb,Cd)S/CdS configuration has superior ability, compared to the conventional PbS/CdS one, to prohibit the interfacial charge recombination occurred at the defect centers of PbS QDs, arising from the effective surface passivation. Consequently, the excellent passivation is responsible for the great improvement of Voc for the (Pb,Cd)S/CdS cell, echoing the results based on J-V characteristics. Aiming to correlate the photovoltaic performance with the loading feature of QDs on the surface of TiO2, we perform a comparison of TEM images between PbS and (Pb,Cd)S QDs sensitized TiO2 nanoparticles, as shown in Fig. 8a and b. Apparently, with the addition of Cd2þ in the cationic precursor, a more homogeneous distribution of QDs with enhanced uniformity of size was achieved. As illustrated in Fig. 8c and d, the addition of Cd2þ ions prevents the excessive growth of PbS QDs in the mesopores, that is, the simultaneous growth of CdS plays a role of confining the subsequent growth of PbS. It is reasonable that the confined QDs with reasonable CB higher than that of TiO2 are beneficial for the efficient electron injection. Since CdS is of a semiconductor with the band gap larger than that of PbS, a type-I heterojunction should be formed at the PbS/CdS interface, leading to the quick injection of photoexcited electrons generated by CdS from its CB to that of PbS [20]. Moreover, as compared in Fig. 8e and f, for the conventional passivation pattern of PbS/CdS, only the surface defects at the contact interface can be passivated, while the direct exposure of a large portion of defect centers deep inside the PbS QD layer might cause severe charge recombination in the solar device in view of the planar-junction-like structure; on the contrary, in our approach, both PbS and CdS are accessible undergoing a simple solutionbased procedure, and the two sulfides interpenetrate with each other intimately, thereby achieving a more effective passivation of trap state defects on PbS QDs with increased passivation area. The CdS herein acts the role analogous to that of capping ligands used in collidal QD photovoltaics. In our approach, one of the key features is the concentration of Cd2þ fed in the precursor, which strongly affects the band-structure and passivation effect. The higher Cd2þ concentration would make it more effective in up-shifting the CB and passivating the defect states for PbS QDs; however, the

negative impact would be the narrowing of the light harvesting range in view of small-sized QDs. Therefore, a reasonable size of PbS QDs and an appropriate amount of CdS passivation layer are desirable for such balance, which can be achieved by optimizing the Cd2þ concentration. 4. Conclusion The present work emphasizes the positive roles of bandstructure tailoring and surface passivation for promising NIR responsive PbS QD-based photovoltaics. A new hybrid configuration of (Pb,Cd)S/CdS, different from the conventional PbS/CdS structure, has been demonstrated for the effective improvement of the photovoltaic performance. Such a splendid configuration scheme not only gives rise to desired charge injection from PbS to TiO2 by constructing a favorable stepwise band alignment, but also contributes to suppressed charge recombination through the passivation of surface defects on PbS QDs. The increased electron injection rate and reduced charge recombination kinetics, coupled with broad light-harvesting capability collectively result in highly efficient QDSC with a PCE up to 4.08% based on (Pb,Cd)S/CdS configuration, achieving significant performance improvement with respect to those of the single PbS (1.06%) or conventional PbS/ CdS (2.95%) systems. Acknowledgements This work was financially supported by the National Natural Science Foundation (NSF) of China (Nos. 51602088, 51372061 and 51302057), the Natural Science Foundation of Anhui Province (No. 1608085QE92 and 1608085ME101), the Fundamental Research Funds for the Central Universities (No. 2016HGTA0699, and 2015HGQC0200) and the China Postdoctoral Science Foundation (No. 2016M590566). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.09.160. References [1] A. Polman, M. Knight, E.C. Garnett, B. Ehrler, W.C. Sinke, Science 352 (2016) aad4424. [2] W. Chen, Y.Z. Wu, Y.F. Yue, J. Liu, W.J. Zhang, X.D. Yang, H. Chen, E.B. Bi, I. Ashraful, M. Gr€ atzel, L.Y. Han, Science 350 (2015) 944e948. [3] J. Du, Z.L. Du, J.S. Hu, Z.X. Pan, Q. Shen, J.K. Sun, D.H. Long, H. Dong, L.T. Sun, X.H. Zhong, L.J. Wan, J. Am. Chem. Soc. 138 (2016) 4201e4209. [4] G.H. Carey, A.L. Abdelhady, Z.J. Ning, S.M. Thon, O.M. Bakr, E.H. Sargent, Chem. Rev. 115 (2015) 12732e12763. [5] M. Kouhnavard, S. Ikeda, N.A. Ludin, N.B.A. Khairudin, B.V. Ghaffari, M.A. MatTeridi, M.A. Ibrahim, S. Sepeai, K. Sopian, Renew. Sust. Energ. Rev. 37 (2014) 397e407. [6] R. Zhou, Q.F. Zhang, J.J. Tian, D. Myers, M. Yin, G.Z. Cao, J. Phys. Chem. C 117 (2013) 26948e26956. [7] R. Zhou, H.H. Niu, Q.F. Zhang, E. Uchaker, Z.Q. Guo, L. Wan, S.D. Miao, J.Z. Xu, G.Z. Cao, J. Mater. Chem. A 3 (2015) 12539e12549. [8] X.Z. Lan, O. Voznyy, A. Kiani, F.P.G. de Arquer, A.S. Abbas, G.eH. Kim, M.X. Liu, Z.Y. Yang, G. Walters, J.X. Xu, M.J. Yuan, Z.J. Ning, F.J. Fan, P. Kanjanaboos, I. Kramer, D. Zhitomirsky, P. Lee, A. Perelgut, S. Hoogland, E.H. Sargent, Adv. Mater 28 (2015) 299e304.  , Q. Shen, H. Zhang, Y. Li, K. Zhao, J. Wang, X.H. Zhong, [9] Z.X. Pan, I. MoraSero J. Bisquert, J. Am. Chem. Soc. 136 (2014) 9203e9210. [10] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Science 348 (2015) 1234e1237. [11] C.eC. Chueh, C.eZ. Li, A.K.eY. Jen, Energy Environ. Sci. 8 (2015) 1160e1189. [12] M. Gr€ atzel, Nat. Mater 13 (2014) 838e842. [13] M.R. Kim, D.L. Ma, J. Phys. Chem. Lett. 6 (2015) 85e99. [14] O.E. Semonin, J.M. Luther, S. Choi, H.eY. Chen, J.B. Gao, A.J. Nozik, M. Beard, Science 334 (2011) 1530e1533. [15] J.B. Sambur, T. Novet, B.A. Parkinson, Science 330 (2010) 63e66. [16] J.B. Zhang, J.B. Gao, C.P. Church, E.M. Miller, J.M. Luther, V.I. Klimov,

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